Introduction: Is There Life On Mars? continued

stellar and nebular, of the sphere belong to one mighty aggregation." This view Clerke still held when Wallace first published his book in 1903, and which, incidentally, Clerke previewed.

By 1925, however, largely due to the work of Edwin Hubble (1889-1953) this view, which had seemed so unassailable, had been replaced with something much grander. The nebulous features long known as spiral nebulae and believed to have been a part of our own Milky Way Galaxy had now been shown to be galaxies in their own right. Hubble had estimated the distance of the Andromeda galaxy (M31) to approach one million light years, well beyond our Galaxy. In 1948 Walter Baade would double both M31's distance from us and its size: in the process our own Galaxy, in terms of size, finally lost its pre-eminent status in the Cosmos. What, however, of other planetary systems and in particular what of life on other worlds? Ironically, despite this new and immensely enlarged vision of the Cosmos in 1925 the prospects for neither of the latter looked good and this was because a few years earlier a new theory of planetary formation took over, one that suggested that planetary formation was rare.

The belief in the abundance of other planetary systems and of extra-terrestrial life, had for a long time been supported by the Kant-Laplace Nebular Hypothesis of planetary formation, which permitted the possible formation of numerous planetary systems from a huge cloud of gas filling the universe (see here). Both Kant and Laplace believed in the possibility of life, even intelligent life, on other worlds: for Kant, in fact, the goal of planets is life. Indeed, it was Percival Lowell’s contention that this theory implied that every planet of our Solar System, physical conditions permitting, was destined to evolve life. By the beginning of the twentieth century, however, this hypothesis had come under attack, and between 1916 and the early 1940's a new theory would predominate: the tidal theory developed by physicist James Jeans. According to this theory planetary systems formed as a result of a close encounter between two stars; since, however, such an event was considered to be extremely rare, so too were planetary systems and in turn, life. So, what of life elsewhere in our own solar system? By the beginning of the 1920's life here did not look too promising either. Mars and Venus were generally regarded as offering the best prospects for life beyond the Earth, but the tide of evidence had begun to turn against them too.

Observations of Venus in 1922 indicated that the planet's atmosphere was devoid of both water and oxygen, so implying the absence of plant life and by 1926 some astronomers had concluded that no life had evolved there. The physical conditions on Venus relied, however, largely upon its rotation rate, which was not finally resolved until about 1967 at 243 Earth days (Dyce:1970). In the meantime figures for the rotation period would vary from one to 225 Earth days, the implications of which for life were quite different at either end of this range, the lower end being the most favourable. Nevertheless, the discovery in 1932 of